Mock 2.0 is using pbr which is hostile to environments without network access.
It's not required on Python 3.5+ so I made it possible to use `unittest.mock`
on this version.
Updated tox.ini to reflect this. Tested with 2.7.11, 3.3.6, 3.4.5, and 3.5.2 on
macOS 10.12.1 with tox and pyenv.
In some cases, when we handle SyntaxErrors we need to ensure that the
column number is correct for a 1-indexed report. In some cases, we also
need to account for the fact that the SyntaxError has happened "after" a
new-line. To extract and alter the row and column numbers, we've moved
the logic to a private static method on the FileChecker object to avoid
an overly complex method.
Closes#237
